Electrolytic operations can include, for example, anodizing, electroplating, electrowinning, electrophoresis, and the like. Basically, an electrolytic bath is housed within an electrolytic cell in which an anode and a cathode are placed. Upon the application of electricity through such electrodes, current is carried by electrolytes in the water (without electrolytes, the water will be subjected to electrolysis). Two oft commercially practiced electrolytic operations will be used to illustrate the precepts of the present invention: electroplating and electrowinning. It will be appreciated that such description is by way of illustration and is not a limitation on the present invention.
Presently, many metals are electroplated on a commercial scale, e.g., aluminum, antimony, bismuth, cadmium, chromium, cobalt, brass, bronze, iron, lead, copper, gold, platinum, rhodium, ruthenium, silver, tin, and zinc. Chromium electroplating, for example, is a widely-used process for depositing chromium metal onto a substrate, typicalily steel for hard chromium electroplating. Chromium offers combined properties not found in any other metal: hardness, high reflectance, high corrosion resistance, low coefficient of friction, high heat conductivity, and excellent wear resistance. Electroplating companies fall into two general categories: captives and job shops. Captive electroplating operations plate in-house manufactured parts and can be found throughout the United States in industries including major airlines, aerospace firms, computer and electronics manufacture, hardware manufacture, automotive companies, and the military. See Freeman (1995), Industrial Pollution Prevention Handbook, McGraw-Hill, New York, N.Y. Freeman also reports that there are about 3,000 job shop electroplating companies in the U.S.
In the chromium electroplating process, a direct current is applied between the anode and the cathode (the part) while suspended in a hexavalent chromium-plating solution. The bath temperature is usually kept at between 116.degree. and 138.degree. F. (46.degree. and 59.degree. C. The bath contains chromic anhydride, which creates an aqueous solution of chromic acid. Sulfuric acid also is present to act as a bath catalyst. At high concentrations (e.g., 225 to 375 g./L of chromic anhydride), the chromic acid forms dichromic acid, which then ionizes to dichromate and hydrogen ions.
Three chemical reactions take place at the cathode (part to be electroplated): (1) the deposition of chromium on the part surface, (2) the evolution of hydrogen gas, and (3) the reduction of hexavalent chromium to trivalent chromium. Three chemical reactions also take place at the anode: (1) the oxidation of the anode, (2) the evolution of oxygen gas, and (3) the oxidation of trivalent chromium to hexavalent chromium.
Chromium electroplating is a very inefficient process in that over 80% of the applied energy goes into the evolution of by-product gases: hydrogen and oxygen. The emission of chromic acid mist from the surface of hard-chrome plating tanks is largely a mechanical process. Hydrogen gas, evolved as a by-product of the redox reaction occurring when plating metallic parts with chromium, bubbles violently out of the solution and causes a boiling action at the surface of the tank. As hydrogen bubbles reach the surface of the tank and burst, a mist composed largely of chromic acid is formed. Additionally, air, often bubbled through the electroplating bath to aid in the mixing of the solution in order to avoid temperature stratification within the bath, also carries chromic acid mist with it as it is evolved from the surface of the tank.
Decorative hexavalent chromium electroplating is similar to hard chromium electroplating, except in: (1) the current applied, (2) the duration of plating, (3) the substrate plated, and, (4) the addition of brighteners and other substances to the bath. A thin layer of chromium is applied to the base material to provide a bright wear and tarnish-resistant surface. Because decorative parts generally are plated at lower currents and for less time then hard chromium electroplated parts, emission generation per surface plated usually also is less. Nevertheless, it is a very significant problem subject to extensive government regulation.
Chromium anodizing is the process of electrolytically oxidizing the surface of a substrate, typically aluminum. An oxidized layer on the surface of the part provides corrosion resistance, low conductivity, and accepting surface for coloring. Although there are different types of anodizing processes, chromium anodizing is preferred because chromic acid acts as a corrosion inhibitor and remains in the pores and crevices of the part after the process is complete. While less of a concern because of short plating cycles, emissions are still a major problem.
The carcinogenicity of hexavalent chromium compounds is well known. Workers involved in chromium electroplating comprise a population at high risk of overexposure to Cr(VI) in the form of chromic acid mist. Chromium and its compounds have been the topic of more epidemiological investigations than any other chemical exposure agent, with the possible exceptions of asbestos and benzene. Lees (1991), "Chromium bands disease: review of epidemiologic studies with particular reference to etiologic information provided by measures of exposure", Environmental Health Perspectives, 92, 93-104. Hexavalent chromium has been shown to cause cancer in humans and in experimental animals, as well as exert genetic toxicity in prokaryotes and mammalian cells in vitro. Norseth (1981), "The carcinogenicity of chromium", Environmental Health Perspectives, 40, 121-130. Under the Clean Air Act Amendments of 1990, the U.S. Environmental Protection Agency (USEPA) has designated chromium compounds as hazardous air pollutants suspected of causing lung cancer in humans. The USEPA has promulgated a National Emission Standard for Hazardous Air Pollutants (NESHAP) that regulates the chromium air emissions generated from chromium electroplating and anodizing operations (60 FR 4948). Additionally, individual states may have different (or additional) regulations. Moreover, most state regulations are more stringent than the national standard and may be based on ground level concentrations or risk-based assessments.
End-of-pipe control technologies have been an accepted method of treating fugitive emissions from the hard chromium electroplating industry. The term "end-of-pipe" denotes the treatment of a contaminated air stream that has been drawn off a plating tank by a blower. Suppressing chromium emissions at the tank level should reduce the amount of chromium at the inlet to end-of-pipe control devices or even eliminate the need for such control devices. Techniques aimed at suppressing chromium emissions from electroplating tanks include chemical foaming agents, small plastic balls, or both used in concert. A study performed by the California Air Resources Board presents data showing that process modifications (specifically, plastic balls, chemical fume suppressants, and elimination of air agitation) will reduce chromium emissions by 50% to 60%. Weintraub, et al., "A systems approach to controlling chrome electroplating emissions", Proceedings of the 34th Annual Meeting & Exhibition, Air & Waste Management Association (AWMA), Jun. 1991, pp 91-103.
Chemical foam blankets provide multiple barrier surfaces with which to collect the mist before being released into the air. Foam blankets have the disadvantage that they can (and often do) trap by-product hydrogen and oxygen gases, thereby forming an explosive mixture. Jordan, Chromium emissions from chromium electroplating and chromic acid anodizing, operations--Background information for promulgated standards, (EPA Publication No. EPA-453/R-94-082b)., Research Triangle Park, N.C.: U.S. Environmental Protection Agency. (NTIS Publication No. PB95166302); and Sheehy, et al. (1984), NIOSH technical report: Control technology assessment: Metal plating and cleaning operations, (DHHS [NIOSH] Publication No. 85-102, Cincinnati, Ohio: National Institute for Occupational Safety and Health, (NTIS Publication No. PB85-234391). Plastic balls (usually polypropylene) of about 30 mm diameter can be floated on the chromium solution to reduce the exposed surface are of the bath and to provide a surface for the mist to deposit on and drain back into the plating solution. However, there is a tendency for the balls to be pushed away from the electrodes by the surface disturbances causes by rising bubbles. Unfortunately, this is the location at which the balls are needed the most to reduce emissions.
Other proposals include, for example, U.S. Pat. No. 3,755,095 which proposes the use of 0.002 to 100 micron size polyethylene powder to reduce chromic acid emissions from the electroplating tank; U.S. Pat. No. 3,657,080 which proposes 0.002 to 100 micron hydrophobic particles (e.g., silica) to reduce chromic acid emissions from the electroplating tank; Russian Pat. No. 1,723,208 which proposes the use of a lower polyethylene granule layer and an upper plastic foam layer; Russian Pat. No. 872,602 which proposes the use of two layers of polymer particles with the top layer pretreated with paraffin wax; and Russian Pat. No. 161,199 which also proposes the use of a combination of 4 mm or smaller polyethylene balls and a chemical foam. David (1946), "Method of reducing chromic acid spray in plating tanks", Safety Review, 3, 13-15, reports a study in which plastic chips were evaluated as a means of reducing chromic acid mist emission from plating tanks, including crushed Lucite crystals measuring approximately 1/4 inch by 1/4 inch, squares of scrap methacrylate, and polystyrene rods measuring approximately 1/4 inch in diameter by 2 inches in length. Davis also reports that in 1926 a German scientist discussed using cork particles or glass wool coated with paraffin to reduce emissions from chrome plating tanks.
Further background information can be found in Hey, "Abatement of Hazardous Air Pollutant Emissions From Army Chromium Electroplating And Anodizing Operations", U.S. Army Construction Engineering Research Laboratories (USACERL), January 1996, (NTIS Publication No. ADA304841) and Fowler (December 1996), An evaluation of the efficacy of styrofoam as a control agent for reducing chromic acid mist emissions from plating tanks in hard-chrome plating operations, Master's thesis, The University of Arizona, Tucson, Ariz. The disclosures of all of the foregoing references are expressly incorporated herein by reference.
Another electrolytic cell process that produces acid vapors and can result in air-borne acid (or salt thereof) and metal above the cell is known as "electrowinning". Electrowinning techniques have been applied to many metals, including copper, gold, lead, and zinc on a commercial scale. By way of example with reference to the electrowinning of copper, basically, electrowinning is a minor ore dressing technique whereby copper ions in an aqueous bath are "plated" out on starter cathodes. One such process practiced commercially leaches copper values from low grade copper ore stockpiles with slightly acidic water to form a "pregnant leach solution" that is extracted with a kerosene-based solvent. The lower raffinate layer is recycled to the ore stockpiles while the Lapper "loaded organic" phase is sent to a stripping tank to be stripped with an electrolyte. After settling, the upper organic phase depleted of copper values is recycled for reuse and the lower "rich electrolyte" is sent to an electrowinning tank house in which tanks are fitted with alternating lead anodes and starter sheet copper cathodes (typically about 38".times.38" (96.5.times.96.5 cm) in size). The bath temperature usually is maintained at about 120.degree.-135.degree. F. (48.9.degree.-57.2.degree. C.). Electrowinning is conducted at rather low currents relative to other plating processes, e.g., 2 v and a current density of 30 amps/ft.sup.2 (1,462 amps/m.sup.2). After several days in the tank house, approximately 250 pound (112.5 kg) copper cathodes are withdrawn and new copper starter sheets are inserted into the baths. The withdrawn copper cathodes are ready for sale or for further processing. Acid vapors are released from the cells and can carry copper metal along with it. Sulfates (including copper) coming off the tanks generally are in the order of 2-10 mg/m.sup.3. OSHA limits are 1 mg/m.sup.3 presently and may be reduced in the near future.
While electrowinning is not a "plating" operation in the traditional sense, it is an electrolytic process that results in a metal being plated from an aqueous acidic bath in an electrolytic cell. Again, like chrome electroplating, electrowinning is another example of an electrolytic cell process that could benefit from having its contents' propensity to be released (aerosolization) from the bath suppressed.